Transceivers are utilized in a number of systems to transmit and receive data and to implement proximity detectors and galvonic isolators. Transceivers utilize a transmitter to send data on a carrier and a receiver to detect the modulated carrier and extract data from that signal. Transceivers based on optical and RF carriers are known to the art.
In a proximity detector, the signal from the transmitter is reflected from an object, and, when the object is within range, the reflected signal is detected by the receiver. The fraction of the transmitted energy that reaches the receiver is usually quite small, and hence, such devices require a significant amount of power and high amplification levels at the receiver.
Galvonic isolators provide a means for moving logic signals between two circuits that must otherwise be electrically isolated from one another. For example, the transmitting circuit could utilize high internal voltages that would present a hazard to the receiving circuit or individuals in contact with that circuit. In the more general case, the isolating circuit must provide both voltage and noise isolation across an insulating barrier. Once again, the fraction of the signal energy that is recovered by the receiver tends to be quite small.
Transceivers that utilize optical and RF carriers are known to the art. An optical transceiver typically includes a light source, such as a light emitting diode (LED) that is used to transmit data by modulating the intensity of the light source and a photodiode that receives the modulated light signals. Optical transceivers operating in the infrared are utilized in computers and handheld devices for transferring data from one device to another without requiring that the devices be connected together by a wire or cable. In such systems, the two devices are positioned relative to one another such that light from the transmitter in the first device is received by the optical receiver in the second device, and vice versa.
Optical transceivers are energy inefficient for two reasons. First, the fraction of the electrical energy that is converted to light is relatively small. Second, the fraction of the light energy that is converted back to electrical energy is also small. Hence, significant amounts of power must be provided at both the transmitter and receiver.
RF transceivers utilize RF signals as the carrier. The transmitter includes a circuit that modulates the carrier and launches the modulated carrier with the aid of an antenna at the transmitter. Similarly, an antenna at the receiver converts the energy in the radio waves to an electrical signal in a conductor that is connected to a receiver that amplifies the signal and recovers the modulation data.
RF transceivers typically operate in the far field. That is, the two antennae are separated by a distance that is greater than a few wavelengths of the RF carrier signal. In this case, the RF transceiver behaves in a manner analogous to that of an optical transceiver. That is, the energy efficiency of the device is small.
In addition, RF transceivers typically operate at frequencies that require relatively large antennae. The efficiency with which the electrical signals from the transmitter are converted to the RF field that couples the transmitter and receiver decreases markedly as the size of the antennae is reduced to lengths that are less than one quarter the wavelength of the carrier signal. Similarly, the fraction of the transmitted energy that is converted to electrical signals in the receiving antenna also decreases markedly as the size of the receiving antenna is reduced to lengths that are less than one quarter the wavelength of the carrier signal. Hence, RF transceivers have not been able to compete with optical transceivers in consumer applications requiring small transceivers and limited power consumption.
Finally, RF transceivers require separate antenna structures that are not part of the integrated circuit in which the transceiver and receiver are fabricated. The need for the separate antennae increases the cost of the transceiver, and hence, further limits the applications that can utilize RF transceivers.